Bike for 5'6" (and growing) 11 yr old?

KTM Mike
i already now i noticed you're katoom fan :)
So your choice should be obvious the only one - go for your son with KTM EXC 200 /especially the '05 model/ which is so lightweight - it's only 2 or 3 pounds of difference between the 125 and 200!
With KDX your son will feel lacking suspension and mostly the weight.
EXC 200 /even older models/ are much more fun factor comparing to kdx and very tractable and easy handful.

Why not a 125 with a big flywheel weight? That will tame it in the short term and you can remove it later when he's ready.

I moved up from a RM80 to a KDX and I love it. I only rode my dads KDX once to know i wanted one. I was about 5' 6-7" then. I'm 5'10" now and my dads 6' and they still fit. You can adjust the shock and forks too. I wanted a TTR instead of my RM at first but i'm glad i got the 2 stroke. The KDX is 230 lbs stock but its got so much low end and handles easily you hardly notice in the trails(unless you drop it in a giant mud hole). Yes the suspension is not brand new so you notice it and the weight on the track(if you push it hard). We were going to get a KX100 and port it but I grew out of it before we could find a good used deal. KDXs are pretty cheap and easily modified to your(or his) his tastes. I'm not a big 'turn a MXer into an enduro bike' fan because it cost more to get a used decent(not thrashed or trashed) MX bike and it cost to modify it. A KTM EXC is nice but the price and limited dealers aren't. If you think a KDX is to big a KTM125EXC or GasGas 125EC would be the perfect bike but good luck finding one.

hope this helps